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Southern California Geology

 

Ground-Subsidence Hazards in Southern California

Subsidence of the ground surface is a geologic hazard that does not have much public visibility-probably because ground subsidence is a phenomenon that typically develops more slowly than other types of geologic hazard. Hence, its effects may not be immediately apparent.

Ground subsidence occurs where underlying geologic materials (typically loosely consolidated surficial silt, sand, and gravel) undergo a change from looser to tighter compaction. As a consequence, the ground surface subsides (lowers). Compaction is a measure of the density of geologic materials. In the case of sediment, the grains of silt, sand, and gravel may be loosely compacted (volume of pore spaces between the grains is large) or tightly compacted (volume of pore space between the grains is small). Where compaction increases (for whatever reason), the geologic materials become more dense. As a result, the ground surface overlying the compacting subsurface materials subsides as the underlying geologic materials settle.

Ground subsidence can occur under several different conditions, including:

 

  • ground-water withdrawal (water is removed from pore space as the water table drops, causing the ground surface to settle)
  • tectonic subsidence (ground surface is warped or dropped lower do to geologic factors such as faulting or folding)
  • earthquake-induced shaking causes sediment liquefaction, which in turn can lead to ground-surface subsidence

Areas where ground subsidence has been observed in southern California include the upper Santa Ana River basin (Chino Valley-San Bernardino Basin-Yucaipa Valley) and the San Jacinto Valley area. Ground-water withdrawal plays a role in ground-subsidence in these areas (Lofgren, 1971, 1976), but in the San Jacinto Valley, tectonic down-dropping of the block between parallel strands of the San Jacinto fault also may have contributed to subsidence (Lofgren and Rubin, 1975; Morton, 1977).

Lofgren, B.E., 1971, Estimated subsidence in the Chino-Riverside and Bunker Hill-Yucaipa areas in southern California for a postulated water-level lowering, 1965-2015: U.S. Geological Survey Open-File Report, 20 p.

Lofgren, B.E., 1976, Land subsidence and aquifer-system compaction in the San Jacinto Valley, Riverside County, California--A progress report: U.S. Geological Survey Journal of Research, v. 4, no. 1, p. 9-18.

Lofgren, B.E., and Rubin, Meyer, 1975, Radiocarbon dates indicate rates of graben downfaulting, San Jacinto Valley, California: U.S. Geological Survey Journal of Research, v. 3, no. 1, p. 45-46.

Morton, D.M., 1977, Surface deformation in part of the San Jacinto Valley, southern California: U.S. Geological Survey Journal of Research, v. 5, no. 1, p. 117-124.


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